1. Field of the Invention
The invention relates to a transmitter and a receiver each of which performs wireless communication with another communication device using a spread spectrum communication mode, and also relates to a wireless communication system and a communication method.
2. Description of the Related Art
Research and development of an advanced safety vehicle (ASV) is being conducted lately. In an advanced safety motor vehicle, the motor vehicle itself is made highly intelligent for the purpose of collision prevention, automatic driving, etc. For example, research about the vehicle-to-vehicle communication is being conducted in order to cope with traffic accidents that occur upon entering a corner, right (or left)-turn accidents or the like, etc.
In the communication that is purposed for collision avoidance, a communication device mounted in a vehicle broadcasts information regarding the position and velocity of the host vehicle, etc. in a time division multiple access mode, and the information is received by a communication device that is mounted in another vehicle. The communication device mounted in the another vehicle performs (1) a partner unit's electromagnetic wave acquisition process, (2) a communication data extraction process, (3) a logic process, and (4) an object extraction process. (1) In the partner unit's electromagnetic wave acquisition process, scanning to search for an electromagnetic wave from a partner unit is performed, and detection of a frequency unique to a communication partner, a timing synchronizing process, and a code correlation process, that is, the despreading of a received signal, are performed. (2) In the communication data extraction process, a demodulation process, a detection process, and a decoding process are performed. (3) In the logic process, data about all the partner units is tabled in accordance with the condition determined by an application installed. (4) In the objection detection process, the partner unit data that matches with various conditions is extracted in accordance with the installed application condition.
For example, in the communication purposed for collision avoidance that is performed by the advanced safety motor vehicles, the communication device mounted in each motor vehicle performs timing synchronization with the communication devices mounted in the other motor vehicles. As a result, the communication device mounted in a vehicle performs, for example, initial synchronism acquisition, and establishes communication with the communication device mounted in a vehicle that is present in a range that the electromagnetic wave transmitted by the communication device of the host vehicle reaches. Then, the communication device of each vehicle transmits data in a time division mode to the communication device with which the timing synchronization has been made. Each communication device transmits data in a communication time slot that is allotted to each communication device. Concretely, each communication device spreads framed data (position information, traveling velocity information) by its unique spread code allotted to the host vehicle, and then transmits the data. The other communication device performs the partner unit's electromagnetic wave acquisition process, the communication data extraction process, the logic process, and the object extraction process as described above. In the logic process, among these processes, the position data and/or the velocity data is tabled separately for each motor vehicle equipped with a communication device. In the object extraction process, for example, the position data and/or the velocity data is sorted so that a vehicle that becomes an object is extracted in accordance with a certain condition. For example, a vehicle that is likely to collide with the host vehicle, concretely, a vehicle which is present around the host vehicle and whose velocity vector intersects with that of the host vehicle is specifically determined. After that, the host vehicle performs communication with the specifically determined value, and avoids the collision by monitoring the movement of the vehicle.
However, the foregoing background art has the following problems.
One of the problems is that it is inevitable for a communication device to execute the communication establishment process with all the partners (communication devices) that the electromagnetic wave from the transmitter reaches. Concretely, the communication device mounted in a vehicle needs to perform communication processes, for example, the foregoing processes (1) and (2), with the communication devices reached by the electromagnetic wave among the communication devices mounted in the vehicles that are present around the host vehicle. Thus, there is a problem of large amounts of communication and large amounts of time required for the communication. Specifically, it takes a considerable amount of time before a vehicle that the host vehicle needs to communicate with is specifically determined. This problem becomes conspicuous when the number of vehicles present around the host vehicle increases. For example, in the case where one frame is constructed of number N slots (N is an integer>0) and the N slots are allotted to vehicles, the position and the traveling velocity of N vehicles can be detected during a period of time when one frame is being transmitted. Therefore, if the number of slots can be increased, the number of detectable vehicles can be increased. However, since there is a limit on the transmission speed that can be secured, the number of slots cannot be increased over a certain number. Besides, in the case where the number of slots is increased, the time taken before the detection of information about the vehicles present around the host vehicle becomes correspondingly long, and the time required before specific determination of a vehicle that is likely to collide with the host vehicle becomes long.
For example, in the case where there are a great number of vehicles, for example, about 5 to 200 vehicles, around the host vehicle in an environment of, for example, an intersection, a two (multi)-level crossing, two-level parallel roads, etc., it is difficult to specifically determine and establish communication with a vehicle that is likely to collide with the host vehicle from the vehicles present around the host vehicle instantaneously, concretely, within several hundred ms, which is permitted by the system. In a currently common communication method with a speed of, for example, several ten bps to 1 Mbps, the establishment of communication requires about several seconds. Therefore, for an application purposed for collision avoidance which is employed in the ASV that is expected to have communication with about 1000 communication units, the communication establishment process consumes an excessively long time.
In order to solve this problem by increasing the communication speed, a level of communication speed that is very hard to realize in the mobile communication is required. Concretely, a communication speed of about 20 Mbps is required. This communication speed is not realistic in the mobile communication, which is affected by various external disturbances, for example, noise, changes in electromagnetic wave intensity. Besides, such high communication speed becomes a factor of cost increase, and is therefore not preferable.
Besides, the communication device mounted in a vehicle needs to perform a process of extracting a partner unit that the application program needs from the communication devices with which communication is possible, among the communication devices mounted in other vehicles present around the host vehicle, for example, the foregoing processes (3) and (4). Thus, there is a problem of large amounts of data and computation processes, and therefore large amounts of time required. These processes have high requirements for hardware resources, for example, the memory capacity, the computation processing capability, etc. Besides, this problem is also a factor of cost increase. Besides, regardless of whether there is a need for communication, these processes are performed after the completion of the process of establishing communication with the communication devices mounted in other vehicles present around the host vehicle. Therefore, a considerable amount of time is taken before the processes are completed.
As described above, with the currently available technologies in the vehicle-to-vehicle communication, it is difficult to detect only a vehicle that is likely to collide with the host vehicle and establish communication with the vehicle within a time that is permitted by the system. For example, in a situation as shown in FIG. 1, it is difficult for a vehicle B to detect only a vehicle C that is likely to collide with the vehicle B and establish communication with the vehicle C. As a measure other than the raising of the communication speed, it is possible to mount a millimeter wave radar in the vehicle B and determine the positional relation with another vehicle by detecting reflection of the electric magnetic wave emitted from the millimeter wave radar. However, it is difficult to detect the direction of the vehicle or the velocity of the vehicle despite the determination of the positional relation with the vehicle. What can be detected by this method is only the relative distance to the vehicle.
Besides, it is also possible to mount a GPS (Global Positioning System) device in the vehicle B and wirelessly notify other vehicles of the position information about the vehicle B. However, this method requires identification codes for discriminating vehicles, and it is difficult to allot all the vehicles with discriminating identification codes. Besides, in the case where a communication system, for example, of cellular phones, is applied, it is also necessary to allot identification codes for discrimination between vehicles. However, since the number of identification codes is limited, it is impossible to allot identification codes to all the vehicles. Although it is conceivable to allot identification codes on a base-station basis, it becomes necessary to arrange an infrastructural facility, thus requiring high cost.
In conjunction with a communication device to which the foregoing spread spectrum communication mode is applied, a spread spectrum receiver that reduces the time needed for synchronization acquisition is disclosed (e.g., see Japanese Patent Application Publication No. 11-145934 (JP-A-11-145934)). This spread spectrum receiver initially acquires a PN signal by a large search step, and then detects the position of a peak in the correlation by a small search step.
However, it is necessary to perform an initial synchronization acquisition with each of the communication devices mounted in all the vehicles present within the range reached by the electromagnetic wave transmitted from the communication device mounted in the host vehicle, and it is impossible to specifically determine a vehicle that is likely to collide with the host vehicle, in addition to establishing communication therewith, within a time permitted by the system.